Patent Application
20110248973
The present invention relates to current-driven display devices such as organic light-emitting diode (OLED) displays, and more particularly to a brightness control drive circuit that automatically adjusts the display device brightness to compensate for changes in incident ambient lighting.
Usage of color display devices in motor vehicle instrument panels and consoles has increased dramatically with the advent of various OEM electronic systems such as navigation, rear-vision, lane guidance, and night-vision, to name a few. Among the more promising display technologies for automotive usage are current-driven display devices such as organic light-emitting diode (OLED) displays, but even these displays have difficulty meeting the required contrast ratio in high ambient lighting conditions unless the display brightness is set to maximum. However, indiscriminately commanding the display to maximum brightness is not realistic, and doing so unnecessarily degrades the life of the display as well. Moreover, the amount of brightness enhancement (if any) needed to compensate for ambient lighting may be different in different areas of the display. Accordingly, what is needed is a selective and cost-effective way of automatically adjusting the brightness of a current-driven display device such as an OLED display to compensate for changes in incident ambient lighting.
The present invention is directed to an improved current-driven display device having sub-pixel drive circuits with integral photo-sensitive circuits that modify the respective sub-pixel drive currents as a function of the locally sensed ambient light level. The photo-sensitive circuit may include a photo-transistor or photo-resistor connected in the output circuit of a drive transistor used to control the on-off state of a respective sub-pixel element, or the photo-sensitive circuit may include a photo-resistor that adjusts a control voltage or current supplied to an input circuit of the drive transistor. In any event, the photo-sensitive circuits individually and independently adjust the drive current, and hence the luminance, of each sub-pixel element based on the locally sensed ambient light level to locally and dynamically compensate for changes in ambient light impinging on the display device.
Referring to the drawings, and particularly to
Referring to
Referring to the first embodiment of
The photo-sensitive circuit 30 is coupled to the output circuit of drive transistor 20, between the drain of drive transistor 20 and the respective power voltage Vdd. The photo-sensitive circuit 30 includes the parallel combination of photo-transistor 32 and resistor 34, the photo-transistor 32 having a collector-to-emitter on-resistance that varies indirectly with incident ambient light level, indicated by the arrows 36. Preferably, the resistance of resistor 34 is selected such that under dark or low-nominal ambient lighting conditions, the sub-pixel element 14 emits light of desired intensity. As the incident light impinging the photo-transistor 32 increases, its on-resistance decreases to proportionately increase the current supplied to sub-pixel element 14 via drive transistor 20 (when biased on). As a result, the intensity of light emitted by sub-pixel element 14 varies in direct relation to the ambient light incident on photo-transistor 32. This effect occurs independently at each sub-pixel element of display 10 so that the display brightness changes locally and dynamically as required to compensate for changes in ambient light impinging on the display device 10.
Referring to the second embodiment of
Referring to third embodiment of
The photo-sensitive circuit 30 is coupled in the input circuit of drive transistor 20, and adjusts the control voltage at node 44 as a function of incident ambient lighting. In this embodiment, the photo-sensitive circuit 30 is configured as a voltage divider, including a photo-resistor 40 coupling the respective data signal Vdata to circuit node 44, and a resistor 42 connected between circuit node 44 and ground voltage Gnd. The resistance of resistor 42 is selected such that under dark or low-nominal ambient lighting conditions, the sub-pixel element 14 emits light of desired intensity. As the incident light impinging the photo-resistor 40 increases, its resistance decreases to proportionately increase the control voltage at node 44, and hence, the intensity of light emitted by sub-pixel element 14 (when Vdata and Vsel are both high). As with the other embodiments, this effect occurs independently at each sub-pixel element of display 10 so that the display brightness changes locally and dynamically as required to compensate for changes in ambient light impinging on the display device 10.
In summary, the present invention provides a current-driven display device having photo-sensitive sub-pixel drive circuits for individually and cost-effectively adjusting the brightness of each sub-pixel element to locally and dynamically compensate for changes in ambient light impinging on the display device. While the present invention has been described with respect to the illustrated embodiment, it is recognized that numerous modifications and variations in addition to those mentioned herein will occur to those skilled in the art. For example, it will be recognized that while the photo-sensitive circuits 30 of this invention have been disclosed in the context of specific sub-pixel driver circuits, the illustrated driver circuits are merely representative of driver circuit topologies that can be used for current-driven display devices, and the photo-sensitive circuits 30 may be equally applicable to other driver circuit topologies. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.
